The great annual Fair of Saint Peter at Shrewsbury, a high point in the city's calendar, attracts merchants from far and wide to do business. But when an unseemly quarrel breaks out between the local burghers and the monks from the Benedictine monastery, a riot ensues. Afterwards a merchant is found dead - and Brother Cadfael is summoned from his peaceful monastery herb-garden to test his skills as a detective once more.